Characterization of the Moloney murine sarcoma virus v-mos protein has been hindered by expression of extremely low levels in transformed cells. In order to analyze the v-mos gene product, a mammalian expression system is being developed in which v-mos protein will be produced at elevated levels. Specifically, plasmid vectors have been constructed in which v-mos has been placed under the control of the murine beta-globin major promoter such that v-mos protein will be synthesized either as a single protein or as a hybrid fusion protein with beta-globin. Plasmid DNA containing v-mos as described above and plasmid DNA containing the gene coding for the enzyme adenine phosphoribosyl transferase (aprt) have been simultaneously introduced into murine erythroleukemia cells lacking aprt (MEL aprt-) using Ca-P04 medicated DNA coprecipitation. Upon induction of differentiation, it is predicted that the exogeneously added beta-globin promoter will be activated to express v-mos protein at elevated levels. This system will provide a method by which to explore the nature of v-mos protein both in vivo, in these MEL cells, and in vitro, after isolation and purification. In addition, such a powerful eukaryotic expression system would be generally applicable for use in studies of other gene products.